FIG. 5 is a cross-sectional view illustrating a prior art inner stripe type semiconductor laser structure in a direction perpendicular to the resonator.
In the FIG., an n type Al.sub.0.5 Ga.sub.0.5 As lower cladding layer 2 is disposed on an n type GaAs substrate 1. An Al.sub.0.15 Ga.sub.0.85 As active layer 3 is disposed on the n type Al.sub.0.5 Ga.sub.0.5 As lower cladding layer 2. A p type Al.sub.0.5 Ga.sub.0.5 As upper cladding layer 4 is disposed on the Al.sub.0.15 Ga.sub.0.85 As active layer 3. A stripe-shaped ridge 9 is formed from the p type Al.sub.0.5 Ga.sub.0.5 As upper cladding layer 4. N type GaAs current blocking layers 5 are disposed, burying the ridge 9 comprising the p type Al.sub.0.5 Ga.sub.0.5 As upper cladding layer 4. A p type GaAs contact layer 6 is disposed on the ridge 9 comprising the upper cladding layer 4 and on the current blocking layer 5. An n side electrode 7 is formed at the rear surface of the substrate 1. A p side electrode 8 is disposed on the contact layer 6. Reference numeral 10 designates an active region and numeral 11 designates a saturable absorption region.
First of all, on the n type GaAs substrate 1, an n type Al.sub.0.5 Ga.sub.0.5 As lower cladding layer 2, an Al.sub.0.15 Ga.sub.0.85 As active layer 3 t.sub.1 thick, and a p type Al.sub.0.5 Ga.sub.0.5 As upper cladding layer 4 are grown by MOCVD or MBE.
Next, side portions of the upper cladding layer 4, except for a portion forming the ridge 9, are removed by etching, leaving a remaining thickness d of the upper cladding layer 4 and, thereafter, an n type GaAs current blocking layer 5 is grown to bury the ridge 9 of the upper cladding layer 4 by MOCVD. Subsequently, a p type GaAs contact layer 8 is grown to cover the ridge 9 of the upper cladding layer 4 and the current blocking layer 5.
Thereafter, an n side electrode 7 comprising Cr/Au or TiPt/Au is formed at the side of the substrate 1 and a p side electrode 8 comprising Cr/Au or TiPt/Au is formed on the p type GaAs contact layer 8, thereby completing the semiconductor laser shown in FIG. 5.
In the semiconductor laser having the above structure, the active layer 3 is divided into the active region 10 and the saturable absorption region 11. The active region 10 is a region into which a current restricted by the current blocking layer 5 is injected and in which laser oscillation occurs. The saturable absorption region 11 has the same structure as the active region 10, but there is no injection of current and it functions as saturable absorber. In other words, when the laser light from the active region 10 is weak, the saturable absorber functions as an absorber to the laser light, and while the light intensity increases to some extent, the absorption of light stops and the saturable absorber functions as a transparent material. Accordingly, the saturable absorber functions as a Q switch for light. By varying the width W of the active region 10 and the remaining thickness d of the upper cladding layer 4 and adjusting the proportion of the light passing from the active region 10 into the saturable absorption region 11, the laser light output can be varied with time as shown in FIG. 6(c), thereby providing pulsation oscillation.
The pulsation oscillation laser has a low coherency of the laser light and has low return light noise and modal noise, thereby serving effectively as an optical disk light source and a high speed LAN (local area network) light source.
In order to produce such a pulsation oscillation laser, it is experimentally well known that it is required to narrow the width W to below 5 .mu.m and to adjust the thickness d to decrease the equivalent refractive index difference .DELTA.n between the active region 10 and the saturable absorption region 11 (normally .DELTA.n .ltoreq.0.01). However, because the allowable range of the width W and the thickness d are narrow and their adjustments are difficult, it is difficult to produce a pulsation oscillation at a high power light output, for example, 10 mW and, it is also difficult to fabricate the pulsation oscillation laser at high yield.